1. Field of the Invention
This invention relates to an image forming apparatus, which forms an image on a sheet by exposing an original to light, such as an electrophotographic copying machine and, more specifically, to an apparatus which allows exposing degree to be adjusted automatically or manually, whichever desired.
2. Description of the Prior Art
As Japanese Patent Publication (KOKAI) No. 59-198471 discloses, conventional copying machines, which allow users to determine whether to set the exposing degree automatically or manually, the standard and the initial exposing degree in manual exposure control mode is set equal to the degree automatically determined by means of exposure lamp voltage and the degree is indicated by the indicator, when the mode is switched from automatic to manual. Such a mechanism is convenient since the standard and the initial degrees in automatic and in manual mode are set to be the same and there is no confusion in exposure control after the mode is switched from automatic to manual.
However, these conventional copying machines have a serious drawback. In case there are seven intermittent exposing degrees to choose from and the automatically determined degree produces the second possible darkest image (step 6), the image can be darker by only one step much (step 7); the range of density choice in manual mode is not wide enough and thus unsatisfactory. It may even be impossible to make the image darker or lighter if the automatically chosen exposing degree corresponds to the darkest or to the lightest of the range, respectively.